1. Field of the Invention
Embodiments of the present invention relate, in general, to mechanisms used to fold wings on an aircraft and more particularly to a two-motion wing-fold mechanism with an independent flight load path.
2. Relevant Background
Many modern-day aircraft, especially military aircraft, are equipped with wing sections that can be folded such that when not spread or deployed for flight operations the wings are placed in a position that allows the aircraft to be stored in an efficient manner. Examples of such aircraft include carrier based airplanes in which the wings fold for compact storage on and below the carrier deck.
Typically, the mechanisms that fold wings must securely lock the wings in the deployed position for flying and, in the case of carrier based aircraft, must lock the wings in the stored position so they do not deploy to spread position due to wind or other forces on the carrier deck. Aerodynamic loads on the outboard wing sections create large moments on such folding mechanisms in both deployed and stored positions. Typically the load experienced by the outboard wing section relative to an inboard wing section is conveyed to the fuselage or inboard section via the same mechanism that acts to rotate the wing into its fully folded or stored position. While the actuating mechanism has the ability to lock in either the deployed or stored position, the load nonetheless is conveyed via the mechanism thus increasing the mechanism's complexity, robustness and weight. In large or heavy military aircraft, such as those found on an aircraft carrier, the added weight with respect to the actuation mechanism needed to withstand such a load as well as fold and store a wing is minimal with respect to the weight of the aircraft. However in smaller, light utility or recreational aircraft, weight of a wing-fold mechanism becomes a significant percentage of the total aircraft weight.
The prior art shows two primary strategies for folding an aircraft's wings. The first is to hinge the wing at a single station and fold the wings so that the wingtips meet above the fuselage or come to rest in a vertical position to reduce lateral displacement. This type of wing-fold can be seen on the F-18 Hornet, the F-4 Phantom and the A-1 Skyraider. A spanwise folding of the wings can result in the aircraft having a large height when wings are in the folded configuration. This raises the center of gravity of the aircraft, which makes it more unstable during ground operations. In addition a spanwise wing-fold mechanism is inherently heavy and complex as a natural upward loading of the wing during flight is in the same direction as the wing-fold loading. Since the flight loads are passed directly through the wing-fold joint itself the mechanism must be designed to be especially robust and heavy.
The second design commonly found in the prior art folds the wings parallel to the fuselage. This technique generally uses a single motion to pivot the wings through an axis oblique to the fuselage or principal axis of the aircraft. This type of wing-fold is embodied in much of the Grumman Corporation's carrier based aircraft. A single motion parallel fold also has undesirable characteristics. A single motion fold requires a section of the upper or lower surface of the wing to be removed or repositioned in order for the outboard folding wing component not to intersect with the stationary inboard section. Single motion folds also arc the wing tip high or low with respect to the fuselage during the folding process. Since ground clearance typically prevents the wing from folding low, the folding mechanism folds the wing high. This motion is not compatible with aircraft having a high T-tail or other structures that place constraints in the wing-fold's path due to possible interference concerns. Furthermore this type of wing-fold can also raise the aircraft's center of gravity making it less stable during ground operations.
Another challenge with the wing-fold mechanisms of the prior art, especially with amphibious aircraft, is reliability and resistance in light of corrosion and general wear and tear. Any mechanism with repeated use will experience a general degradation in performance. Frequent exposition to water as would be experienced by an amphibious aircraft heightens this degradation. To compensate for such degradation the wing-fold mechanisms are typically over engineered resulting again in increased weight and complexity.
The need therefore remains for a single wing-fold mechanism that is both simple and lightweight and that separates the wing-fold actions from the in-flight wing loading. These and other improvements of the prior art are hereafter described by way of example.